How to find the eigenvalues ​of the following block matrix $ \pmatrix{0&X\\ X^T&A} $?

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Assume that $X$ is an $1 \times n$ matrix. Also, $A$ is an $n\times n$ matrix. How to find the eigenvalues ​​of the following block matrix $ \pmatrix{0&X\\ X^T&A} $?

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Call the matrix $Y$. If $Y$ has a zero eigenvalue, its determinant is zero. A nonzero eigenvalue $\lambda$ would satisfy

$$0=\text{det}(Y-\lambda I_{n+1})=\text{det}\left(\begin{array}{cc} -\lambda & X\\ X^{T} & A-\lambda I_{n} \end{array}\right)=-\lambda\text{det}\left(A-\lambda I_n+\frac{1}{\lambda}X^{T}X\right)\\ \implies 0=\text{det}\left(\lambda A-\lambda^2 I_n+X^{T}X\right),$$

using the block matrix determinant identity.